Answer: Salivary duct carcinoma

Histology: The parotid parenchyma is infiltrated by rounded nodules of tumor. Architecturally, the tumor nodules have central cystic spaces that are lined by tumor cells forming cribriforming structures. The nodules are expanded by central zones of prominent cellular necrosis. The appearance of these nodules is reminiscent of intraductal comedo carcinoma of the breast. The cytoplasm of the tumor cells is distinctly eosinophilic. The nuclei are round to oval with prominent nucleoli.

Discussion: Salivary duct carcinoma is a rare and highly aggressive salivary gland malignancy. Most of these tumors arise in the parotid gland, and patients often present with a rapidly growing mass and facial nerve dysfunction. Patient prognosis is very poor.

The tumor is mainly recognized by the way it resembles intraductal carcinoma of the breast. The cystic structures are reminiscent of mammary ducts, and the tumor often forms cribriform, papillary and solid patterns within these duct-like spaces. The presence of comedo necrosis is a common finding. Although these cystic structures resemble an intraductal process, these structures actually represent nodules of infiltrating carcinoma. Accordingly, “intraductal carcinoma” is not an appropriate synonym for salivary duct carcinoma.

The tumor cells frequently are characterized by abundant pink cytoplasm. Not surprisingly, the differential diagnosis encompasses the group of salivary gland carcinomas marked by abundant pink cytoplasm including oncocytic carcinoma. Salivary duct carcinoma is distinct from these other tumor types at the architectural level. The presence of duct-like spaces with cribriform growth and comedo necrosis is not typical of oncocytic carcinoma. Adenocarcinoma N.O.S. is a general term that is reserved for those tumors that are not easily pigeonholed into a more specific category. The possibility of a metastatic breast carcinoma should be excluded by a careful clinical history. Of note, salivary duct carcinoma can express some markers usually associated with breast cancer such as Her2 and gross cystic disease fluid protein. Androgen receptor is expressed in most salivary duct carcinomas and in a minority of breast carcinomas.